There is a growing need for a flat-panel display as a display device of a computer with a wide spreading use of information terminals. With further advancement of informatization, there are also increasing opportunities in which information, which has been conventionally provided by paper medium, is digitized. Particularly, the needs for an electronic paper or a digital paper have been recently increasing since they are thin and light weight mobile display media which can be easily held and carried (see Patent document 1, described below).
Generally, the display medium of a flat panel display device is formed by using an element such as a liquid crystal, an organic EL (organic electroluminescence) and an electrophoresis. In such display medium, a technology which uses an active drive element (TFT element) as an image drive element has become a mainstream in order to secure a uniformity of the screen luminosity and a screen rewriting speed and so forth. In the conventional computer display device, TFT elements are formed on a glass substrate, and a liquid crystal element, an organic EL element or the like is sealed.
As the TFT elements, semiconductors including a-Si (amorphous silicon) and p-Si (polysilicon) can be mainly used. These Si semiconductors (together with metal films, as necessary) are multilayered, and also each of a source electrode, a drain electrode and a gate electrode is sequentially stacked on a substrate, which leads to an achievement of the production of the TFT element.
The conventional method of manufacturing a TFT element using Si materials includes one or more steps with a high temperature, so that there is an additional restriction that the material of the substrate should resist a high process temperature. For this reason, it is required in practice to use a heat-resisting glass as the material of the substrate. In the meanwhile, it may be possible to use a quartz substrate. However the quartz substrate is so expensive that an economical problem arises when scaling up of the display panels. Therefore, a glass substrate is generally used as a substrate for forming such TFT elements.
When the thin display panel as described above is produced by using the conventionally known glass substrate, there is, however, a possibility that such display panel has a heavy weight, lacks flexibility and breaks due to a shock if it is fallen down. These problems, which are attributable to the formation of a TFT element on a glass substrate, are so undesirable in light of the needs for a portable thin display having light weight with the advancement of informatization.
From the standpoint of obtaining a substrate having flexibility and light weight to meet the needs for a lightweight and thin display, there is developed a flexible semiconductor device wherein TFT elements are formed on a resin substrate (i.e., plastic substrate). For example, Patent document 2 (see below) discloses a technique in which a TFT element is formed on a substrate (i.e., glass substrate) by a process which is almost the same as conventional process, and subsequently the TFT element is peeled from the glass substrate and then transferred onto a resin substrate (i.e., plastic substrate). In this technique, a TFT element is formed on a glass substrate and the TFT element together with the glass substrate is adhered to a resin substrate via a sealing layer (e.g., an acrylic resin layer), and subsequently the glass substrate is finally peeled thereoff. As a result, the TFT element can be transferred onto the resin substrate.
In the method for manufacturing a TFT element using such transference process, there is, however, a problem in the peeling step of the substrate (i.e., glass substrate). In other words, it is necessary to perform an additional treatment to decrease the adhesion between the substrate and the TFT element upon peeling the substrate from the resin substrate. Alternatively it is necessary to perform an additional treatment to form a peel layer between the substrate and the TFT element and to physically or chemically remove a peel layer afterward. These additional treatments make the process complicated, so that another problem concerning productivity is caused.